CS 241: Systems Programming Lecture 30. Dynamic Libraries Fall 2019 - - PowerPoint PPT Presentation

CS 241: Systems Programming Lecture 30. Dynamic Libraries

Fall 2019

• Prof. Stephen Checkoway

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Announcements

No class on Friday No office hours Thursday or Friday of this week

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Last time

Static libraries (or archives) are a way of bundling a collection of object files together

• Use the compiler to create .o files
• Use ar to create .a file
• For each program, we want to create, use the .a at the end of the link

line
 $ clang -o prog main.o libfoo.a

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Dynamic libraries

Like static libraries, dynamic libraries start as a collection of object files (.o)

• When linking an executable against a static library, the program linker

copies the relevant library code/data into the output Unlike static libraries, dynamic libraries are produced by the linker

• When linking an executable against a dynamic library, the program

linker inserts references to the library into the output, but does not copy the library code/data into the output At run time the dynamic linker (the loader) loads the executable and all of its required libraries into memory

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Static library

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library .o files program .o files

prog libex.a

ar ld

Dynamic library

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library .o files program .o files

prog

ld ld

libex.so

Reference

Differences at runtime

Programs linked to static libraries

• Library code/data is part of the program
• Only the object files needed are included
• Code/data is (usually) placed at a known fixed address
• Each such program has its own copy of the code/data

Programs linked to dynamic libraries

• Library code/data is loaded into memory separately
• The whole library is included, not just the needed bits
• Library code/data is loaded at an unknown address
• Multiple programs can share a single copy of library code and read-only

data; they need their own copy of the writable data

• The program loader needs to perform more work at program start up

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When a library is used by many applications (e.g., libc), which of the following is not a benefit of using a dynamic library as compared to using a static library?

• A. Smaller memory usage for an individual application
• B. Smaller total memory usage across multiple applications
• C. Smaller total disk usage across multiple applications
• D. Faster program linking

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When a library is used by only one application, which of the following is not a benefit of using a static library as compared to a dynamic library?

• A. Smaller memory usage for the application
• B. Smaller disk usage for the application
• C. Faster program startup
• D. Better program performance (it runs faster) separate from its start up

speed

• E. Bugs in the library can be fixed independently of the application

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Creating a foo shared object

Steps

• Object files need to be compiled as position-independent code (PIC)


$ clang -fPIC -o a.o a.c

• The compiler/linker needs to be informed that it's producing a shared
• bject with a given soname (see next slide)


$ clang -fPIC -shared -Wl,-soname=libfoo.so.1 \


• o libfoo.so.1.0.0 *.o

Option details

• -fPIC — produce position-independent code
• -shared — produce a shared object
• -Wl,-soname=blah — pass -soname=blah to the linker

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soname (ELF-based systems)

Each dynamic library has a soname

• lib⟨name⟩.so.⟨ABI version⟩
• ABI is application binary interface
• The soname specifies the name of the library and its ABI version
• Multiple versions of a library with a compatible ABI have the same

soname

• Versions of a library with incompatible ABIs (different functions or

parameters) have a different soname

• libc.so.5
• libc.so.6

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soname vs. file name (Linux)

Example sonames

• zlib (a compression library) has the soname libz.so.1
• libc's soname is libc.so.6
• PCRE's library's soname is libpcre.so.3

On the file system the soname is a symbolic link to the actual library

• The file name is usually lib⟨name⟩.so.⟨major⟩.⟨minor⟩.⟨patch⟩
• The major version number is often the ABI version
• libz.so.1 -> libz.so.1.2.11
• libpcre.so.3 -> libpcre.so.3.13.3
• libc.so.6 -> libc-2.27.so <- Nonstandard name!

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One additional symbolic link

For a given library foo, there are typically two symbolic links

• libfoo.so -> libfoo.so.1.0.0
• libfoo.so.1 -> libfoo.so.1.0.0

The first symbol link is used at link time, the second at run time The two need not be in the same directory

• /usr/lib/x86_64-linux-gnu/libz.so -> 


/lib/x86_64-linux-gnu/libz.so.1.2.11

• /lib/x86_64-linux-gnu/libz.so.1 -> libz.so.1.2.11
• /lib/x86_64-linux-gnu/libz.so.1.2.11

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Linking to a .so -l (lower case L)

We can also specify a library using a command line option: -l

• $ clang -o prog main.o -lfoo

Using -lblah tells the linker to look for the file

• libblah.a

— a static library

• libblah.so

— a dynamic library on ELF-based systems

• libblah.dyld — a dynamic library on macOS

libblah.so is a symlink to libblah.so.1.0.0 which has a soname of libblah.so.1

• The compiler records libblah.so.1 in the output prog

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Compiler search paths

When the compiler searches for files, it looks in a variety of paths

• Header files come from the header search path
• Library files come from the library search path

We can add a directory to a specific search path

• Headers: -Ipath (e.g., -Iinclude)
• Libraries: -Lpath (e.g., -Llib)

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Example

We have a library, foo, we want to link against with

• headers in foo/include
• libraries in foo/lib

We add

• -Ifoo/include to CFLAGS
• -Lfoo/lib -lfoo to LDFLAGS

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Runtime search paths

When the program starts, the dynamic linker looks at the sonames recorded for all of the binaries and looks for a file with a matching name (which is usually a symlink) and loads that library An additional runtime path can be added to the program at link time by using -Wl,-rpath=⟨path⟩ to add path to the list of directories searched By using the special symbol $ORIGIN we can add a path relative to the directory of the program

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In-class exercise

https://checkoway.net/teaching/cs241/2019-fall/exercises/Lecture-30.html Grab a laptop and a partner and try to get as much of that done as you can!

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